The most common windmill employs a single propeller rotating about a horizontal axis. A normal fraction of wind energy available from air flow to this type windmill is controlled by the interaction of the propeller with the ambient wind field and is limited by what is known as the Betz limit to 59% of the unperturbed wind energy over the area of rotation of the propeller. This limitation is for optimum design, and, practically, with most conventional two-bladed propellers, the efficiency factor is typically no greater than 40%. In an effort to provide improved efficiency, various augmenting structures have been proposed.
As most pertinent to the present invention, it has previously been proposed, as illustrated in U.S. Pat. No. 4,087,196, that two concentrically positioned propellers be employed, a large one and a small one, and that these be disposed at opposite ends of a conically-shaped shroud. Air enters the larger end of the shroud and is, in accordance with the patent, increased in velocity by an amount equal to the ratio of the outlet and inlet areas. The small propeller is then driven by air with this enhanced velocity. It, in turn, operates a generator, and, in one example, drives through a reduction gear the larger propeller at the inlet end of the shroud. Alternately, the inlet propeller may be driven by other means. The basic problem with employing a coupling shroud between propellers is that the shroud must be very large, and thus heavy and expensive, both to construct and to mount.
It is the object of the present invention to provide a windmill with enhanced efficiency, and at the same time one which employs certain smaller and less costly components than previously proposed.